Silicon single crystal that is mainly produced by a Czochralski (CZ) method is sliced into silicon wafers to be used as substrates for semiconductor devices such as memories or image sensors. This silicon single crystal produced by the CZ method contains oxygen atoms. In fabrication of devices with the silicon wafers sliced from such silicon single crystal, silicon atoms and oxygen atoms are combined to form oxide precipitates and bulk micro defects (BMDs).
It is known that these oxide precipitates and BMDs provide intrinsic gettering (IG) capability to capture contaminant atoms such as heavy metal atoms in the interior of wafers and thereby to improve device characteristics. Wafers having a larger amount of oxide precipitates or a higher BMD density in its bulk allow high-performance devices with high reliability to be obtained.
In recent years, there is a need for semiconductor devices having radiation resistance that use silicon wafers doped with phosphorus. Use of substrates having a high oxygen concentration for such devices enables great improvement in device characteristics.
In production of a single crystal, an oxygen concentration is controlled during its growth so as to be taken in the crystal at a high concentration, in order to give these semiconductor devices useful IG capability and a high oxygen concentration.
There is also an increasing need for various wafers: polished wafers obtained by performing mirror polishing on silicon wafers sliced from a silicon single crystal produced in the above manner; annealed wafers obtained from the polished wafers subjected to an annealing process to suppress defects in their surface layer or to form an intrinsic gettering (IG) layer in their bulk; epitaxial wafers each having a formed epitaxial layer; and SOI wafers.
Since these wafers are subjected to multistage device processes, there are factors in impeding electrical characteristics: metallic impurities intrude into device regions during these processes; intrusion of radial rays or cosmic rays into device regions gives rise to ionization. It is accordingly an essential problem to advance a technique to prevent spread of harmful impurities such as an uneven image for an image sensor, or to neutralize the ionization due to radial rays or cosmic rays for a device having radiation resistance. A technique to pull a crystal that particularly is doped with phosphorus and has a high oxygen concentration of 1.6×1018 atoms/cm3 (ASTM'79) or more is requested for use in some image sensors and devices having radiation resistance.